Receivers for wireless communications, which may be incorporated into a base station (BS) or a user equipment (UE), and the like and operate in accordance with multiple standards, are generally equipped with an AGC function, as depicted in FIGS. 1 and 2, which respectively illustrate schematic block diagrams of an analogy part 10 and a digital part 20 of a receiver for performing AGC.
The processing chain of the analogy part 10, among other things, includes a radio frequency (RF) filter, a controllable attenuator, an intermediate frequency (IF) filter, a variable gain amplifier (VGA), an IF mixer, an anti-aliasing (AA) filter, and an analog to digital converter (ADC). Upon completion of the processing of the analogy part 10 of the receiver, the received signal would come into the processing of the digital part 20 of the receiver. As illustrated, having been subjected to the processing of the analog part 10, the resulting digital signal enters into the digital part 20, in which in addition to the conventional processing including a fast fourier transform (FFT) decoding, and Eb/I0 reading, the power level of the received signal would be measured by a power meter and then compared with a number of predefined thresholds. Then the receiver would apply the AGC setting resulting from the digital part 20 to the analogy part 10, as illustrated in FIG. 1 by a dashed line, based on the result of the comparison of the measured power level with the predefined thresholds.
The AGC is advantageously used to ensure that the signal received by the receiver should be processed at a suitable signal level (e.g., suitable signal power) by correspondingly adjusting the gain of the receiver in response to varying input signal levels. For example, the AGC scales the received signal such that the signal can be represented by a limited number of bits for digital processing without significant loss of information. The receiver gain determined by the AGC should be large enough to minimize quantization noise. In the meantime, the AGC should ensure that the power of the scaled signal does not exceed a maximum range of the digital processing, i.e., the digital part 20 does not get saturated.
FIG. 3 is a schematic illustration of active periods and deactivate periods of the above-discussed AGC in a time division duplex (TDD) frame. As seen from FIG. 3, during the time interval including time periods of a subframe #2 and a subframe #3, which are scheduled to transmit on the uplink (UL), the AGC keeps active. In other words, during the reception period in which a BS plays a role as a receiving party, the AGC retains active mode unchanged. In contrast, during the time interval including time periods of a subframe #4 and a subframe #5, which are scheduled to transmit on the downlink (DL), and a special subframe, which includes a DwPTS part, a guard period (GP) part, and an UpPTS part, the AGC keeps deactivated. That is, the AGC retains deactivate mode unchanged during the transmission period of the BS.
Simply put, in the UL subframes, the AGC function is continuously active based on the instantly detected received power level while in the DL subframe and special subframe, the AGC function is deactivated and determines a constant receiver gain setting so as to keep the receiver gain unchanged or maximize achievable attenuation.
In the UL subframe, when the received power level reaches one of multiple predefined thresholds, such as those illustrated in FIG. 2, the AGC would be trigged and thus the gain setting of the receiver would be updated. In other words, different received power levels may trigger multiple AGC states when comparing with the predefined threshold. Due to different states, the receiver gain would be correspondingly set to a different gain setting.
In view of the foregoing discussions made with respect to FIGS. 1-3, it can be understood that the current AGC is a complete passive protection method for received high power level interference and is only triggered by incoming high power level interference, resulting in some drawbacks as discussed below.
First, the AGC reaction needs extra reaction time to update the gain setting in the receiver and this reaction time may occupy the normal receiving time periods, thereby adversely impacting the normal receiving operations.
Second, the rapid changes in the signal level and phase will generate spurious effects (also known as “glitch”) during transitions through different gain/attenuation paths in the receiver when the AGC changes the receiver gain setting, which is shown in FIG. 4, which is a schematic illustration of glitches upon AGC gain changes (decrement or incensement) or receiver gain setting changes. During the glitch period, as depicted by “tglitch_dur,” the normal receiving operations cannot be properly performed and the processing in the baseband will be affected in a negative way. The glitch suppression method implemented in the common signal processing is to simply insert zero data instead of the samples affected by the glitch. Generally, the AGC needs extra time to re-stabilize the receiver gain after the interference signal injects into the receiver. Lots of glitches spread in time can cause receiver efficiency to decrease rapidly and impact throughput seriously. In worst case, wireless communications could be temporarily interrupted or may even break down.
For instance, in a TDD communication system in which the UL subframes are not continuous, the proportion of wasted UL time period resources is higher than what is in the FDD system due to the glitch and consequently the glitch impact to system performance becomes more critical in complex application scenarios.
In particular, in the TDD system, all possible UL/DL subframe configurations are exemplarily shown in table 1.
TABLE 1DL to ULUL-DLswitch point Subframe numberconfigurationperiod012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
It can be considered that configuration 5 is the worst case from an AGC point of view because it merely has one UL subframe (i.e., subframe 2). In all else subframe time intervals of 9 ms (1 ms per subframe), instant interferences are complex and changeable. Due to this, the AGC setting in the last UL subframe almost cannot be a correct reference for AGC setting in the next UL subframe. At the same time, due to lack of UL resource for AGC detection, correct recovery of normal receiving takes rather longer time. In the TDD mode, it takes time duration of more than one orthogonal frequency division multiplexing (OFDM) symbol for AGC gain convergence, which results in a significant glitch effect.
There are proposed some existing solutions for addressing some drawbacks in the AGC. For example, in the paper entitled “A Fast Automatic Gain Control Scheme for 3GPP LTE TDD System” in Vehicular Technology Conference Fall (VTC 2010-Fall), 2010 IEEE 72nd, a fast AGC circuit is proposed, wherein an average signal amplitude ratio of the received sample signal is applied after the ADC to trigger the AGC, in order to avoid large signal power variation. The problem with this solution is that the average signal amplitude ratio could bring about a risk of instant burst interference, which cannot be detected by the AGC function and it needs a total new digital architecture, which is not a common and easy design. Further, in the U.S. Pat. No. 8,144,634B2, entitled “reducing automatic gain control process in time division duplex communication mode,” the noise level detected in the DwPTS or GP part of a special subframe is proved to be used as input of the AGC setting in the next UL subframe. The problem associated with this patent is that an additional hardware circuit is needed for detection of the noise level so as to build a new AGC feature and extra ADC and digital resources are requisite for processing the detected noise signal. It would be more challenged to achieve high accuracy of noise detection circuit with low cost and the noise detection circuit with high accuracy would incur more undesirable hardware cost and consume more potential resources.